As textile substrates age, their color tends to fade or yellow due to exposure to light, air, soil, and natural degradation of the fibers that comprise the substrates. As such, to visually enhance these textile substrates and counteract the fading and yellowing the use of polymeric colorants for coloring consumer products has become well known in the prior art. For example, it is well known to use whitening agents, either optical brighteners or bluing agents, in textile applications. However, traditional whitening agents tend to lose efficacy upon storage due to deleterious interactions with other formulation components (such as, for example, perfumes). Further, such whitening agents can suffer from poor deposition on textile substrates. As such, formulators tend to increase the level of whitening agent used to counteract any efficacy lost upon storage and/or to increase the amount of whitening agent available to deposit on the textile substrate.
Leuco dyes are also known in the prior art to exhibit a change from a colorless or slightly colored state to a colored state upon exposure to specific chemical or physical triggers. The change in coloration that occurs is typically visually perceptible to the human eye. Many of these compounds have some absorbance in the visible light region (400-750 nm), and thus more or less have some color. In this invention, a dye is considered as a “leuco dye” if it did not render a significant color at its application concentration and conditions, but renders a significant color in its triggered form. The color change upon triggering stems from the change of the molar attenuation coefficient (also known as molar extinction coefficient, molar absorption coefficient, and/or molar absorptivity in some literatures) of the leuco dye molecule in the 400-750 nm range, preferably in the 500-650 nm range, and most preferably in the 530-620 nm range. The increase of the molar attenuation coefficient of a leuco dye before and after the triggering should be greater than 50%, more preferably greater than 200%, and most preferably greater than 500%.
Leuco compounds can be used as whitening agents in laundry care compositions (e.g., laundry detergents). Leuco colorants are effective in such application to the extent that they maintain a colorless form on storage in a detergent and undergo a triggered change to a colored or much more highly colored state during or after use by the consumer. The challenge is to keep the leuco in its uncolored form over potentially long storage periods, yet have the leuco converted through a triggering mechanism upon use by the consumer. Various means could be envisioned to do this, such as adding a separate oxidant to the wash to convert the leuco to its colored form, but consumers do not normally prefer added complexity in their routine laundry processes. It is also possible that residual oxidant remaining in the water used to do laundry may convert some portion of the leuco colorant, but this is effective only for those consumers who have the appropriate water source. The need remains, therefore, to enable a consumer to enjoy the whitening benefits that come from a leuco dye without the added complexity of specialized products that need to be used in conjunction with the main detergent or where the water source is sanitized by a means that is not effective at conversion of the leuco.
This need can be met by developing leuco colorants that deposit at sufficient levels through the wash because we have surprisingly found that certain leuco colorants, when deposited on fabric, convert to the colored form over time simply upon drying and subsequent standing, without need for the intentional addition of an oxidant to accelerate the process. This means the consumer can enjoy the benefits of whitening made possible by the fully oxidized form simply by designing the leuco colorant to be one that both deposits through the wash and then subsequently converts to the colored form upon drying and standing.